In an image reproducing system, a beam modulated by the density of each portion of an original picture is detected by a photomultiplier as the corresponding variation of the output current thereof and is input to an analog color computation device as a density signal after undergoing current-voltage conversion.
In the above situation, the reference voltage of the color computation device is rendered to correspond to the white level signal from the photomultiplier. While the maximum incident quantity of the input scanning beam being introduced to the photomultiplier varies according to the size of the pickup area to be scanned or the type (size) of the optical aperture to be used for performing detail adjustment (the aperture is built in a cartridge type aperture box). Therefore, in order to compensate the variation, the sensitivity of the photomultiplier must be adjusted in each case, which work is called a "basic calibration".
Conventionally, the basic calibration is performed by using a servo mechanism as shown in FIG. 1. That is, at first the anode current of the photomultiplier 10 is converted into the corresponding voltage V.sub.a in an I/V converter 1. Secondly, the output voltage V.sub.a and the reference voltage V.sub.b of the color computation device are input to a differential amplifier 2. Then according to the output of the differential amplifier 2, a cathode resistor R.sub.1 (a potentiometer) or one of dynode resistors D.sub.y1 to D.sub.yn+1 (usually the second resister R.sub.2 (a potentiometer)) is adjusted by a servo-motor M.sub.1 (for the resistor R.sub.1) or M.sub.2 (for the resistor R.sub.2). However as the resistors R.sub.1 and R hd 2 are potentiometers, the variable range of each of them is comparatively narrow, which means they can be adjusted only in a limited range. Therefore, a variable resistor VR.sub.1 and a resistor selector composed of a switch SW.sub.1 and resistors R.sub.4 and R.sub.5 are added to the cathode resistor R.sub.1, thereby rough adjustment is performed by the resister selector and the variable resister VR.sub.1, and minute adjustement is performed by the resistors R.sub.1 or R.sub.2. However this kind of system has too many parts to be adjusted, which also leads to a troublesome handling.
Furthermore, there are employed usually four to six photomultipliers in a color scanner for reproducing plural kinds of color separation films, and said basic calibration work must be carried out on all the photomultipliers for render them to have the same sensitivity. In this case, since they must have the same sensitivity under a specific condition (for example, the condition in which an aperture of a certain size is used and the resistor R.sub.4 is selected), sometimes the difference of sensitivity between the photomultipliers cannot be compensated by said servo system alone when the difference exceeds the controllable range of the system. So, in order to overcome the above drawback, photomultipliers of the same sensitivity characteristic are preferable for that use, however, pursuit of which also ends in inefficient yield and consequent high cost of producing photomultipliers.
On the other hand, if said aperture box is pulled out from the head leaving the high voltage be applied to the cathode of the photomultiplier, an intense beam rushing into the photomultipliers destroys it. So to avoid the trouble, an operator must cut off the cathode voltage before pulling out the aperture box from the head. Providing an OFF terminal 4 to the switch SW.sub.1 and thereby selecting the terminal is a resolution to the problem indeed, however which becomes meaningless when neglected.
In addition, a system thus constructed comprises a certain number of mechanical contact points in such as the switch, the variable resistors and the servo potentiometers. And as the contact points are usually given a negative potential of several hundred of volts, static electricity charged on them easily attracts dusts, which also causes troubles. Moreover the servo potentiometer has a structure that the revolution shaft of a potentiometer is revolved by a small servo motor, and it sometimes snaps or goes wrong. And in a color scanner, every photomultiplier needs at least one servo potentiometer(s), which pushes up the cost of the scanner system.